1. Field of the Invention
The subject matter disclosed generally relates to the field of heat dissipation devices for electronics components.
2. Background Information
Many modern electrical devices, such as computers, include integrated circuits having many small circuit components that are fabricated on a fragile chip or “die.” Practical circuit components generate heat during operation. The complexity, compactness, and fragility of the circuitry on the die presents several challenges to designers of heat dissipation devices for the die. First, the heat dissipation device must facilitate adequate thermal egression to prevent an associated die from reaching an excessive temperature. Second, the heat dissipation device must be electrically isolated from the circuitry on the die so as not to interfere with the operation of the circuit. Third, the heat dissipation device must provide adequate rigidity to allow the fragile die to be handled without being damaged in subsequent manufacturing or assembly steps, such as attachment of a heat sink.
One conventional strategy intended to meet the above challenge is to encapsulate the die in an epoxy or other encapsulant material, which fills the space between a heat spreader cap and an underlying substrate board to which the die is electrically connected. An advantage of this conventional strategy is that a thick layer of encapsulant surrounds and protects the die and also supports and attaches the heat spreader cap. Because the encapsulant layer supporting the heat spreader cap is thick, it is relatively compliant in shear and so does not present an edge constraint on the thermal expansion of the cap relative to the substrate board. Thermally induced stresses due to the mismatch of coefficient of thermal expansion (CTE) between the cap and the die produce stress near the center of the cap, but stresses that would be caused due to CTE mismatch between the cap and the substrate board are relieved by shearing of the thick encapsulant layer. One reason why this is important is because, typically, the temperature difference between the die and the substrate board is larger than the temperature difference between the die and the cap.
However, the conventional strategy of chip encapsulation suffers from practical problems and assembly difficulties associated with the control of a relatively large volume of pre-solidified encapsulant in a mass-manufacturing environment. A conventional strategy to reduce these manufacturing difficulties is to eliminate the thick encapsulant layer and instead support and attach the heat spreader by forming a lip around its periphery that is bonded directly to the substrate board by a bonding layer that is significantly thinner than previously used encapsulant layers. Today, such a bonding layer might have a thickness less than 0.25 mm. Such a conventional heat spreader typically has a cavity in its center region that attaches to the die and that is raised up from the packaging substrate. The die is thermally coupled with the heat spreader lid by either direct contact or via a thermal interface material situated between the die and the inner surface of the cavity in the lid. The raised peripheral lip, formed around the cavity, extends down toward the substrate board and is bonded to the substrate board. A metal plate called a heat sink, which may include metal fins to increase heat transfer to the surrounding air, is typically bonded, perhaps by a thermally conductive adhesive, to the typically flat, upper surface of the heat spreader lid.
Typically the die, substrate, heat spreader lid, and heat sink do not all have the same coefficients of thermal expansion. Differences in the coefficients of thermal expansion can lead to the creation of significant stresses during operation. Such stresses can be expected to cyclically vary because all the components are typically subjected to temperature cycling during operation of the device. For example, semiconductor packages that have been soldered to a motherboard and placed in a computer are typically subjected to temperature cycling during computer operation. An example of such temperature cycling may be a temperature variation between the environmental temperature (e.g. room temperature) and the maximum electrical component operating temperature (e.g. 100° C.). Resulting periodic variations in thermal stress may ultimately result in functional and/or structural failure, including cracking of the die, adhesive failure between the lid and the substrate board, and/or separation between the lid and the thermal interface material thermally coupling it to the die. For example, cyclically varying thermal stresses can cause periodic warping of the heat spreader lid or underlying substrate board, which, in turn, can cause the thermal interface material to migrate from the gap between the die and the heat spreader lid. Such migration (or pumping) of the thermal interface material can cause an undesired increase in the thermal isolation between the die and the heat spreader lid, and therefore can cause functional failure.
The heat spreader lid material typically has a greater coefficient of thermal expansion than the underling substrate board. Therefore, as temperature increases, the lid expands and its deformation is constrained by the bonding of the edge or “lip” of the lid to the substrate. As the lid expands, it may warp in such a way that the center portion of the lid's cavity rises away from the die, or it may cause the substrate board to warp. Such warping degrades the interface between the lid and the die, impeding heat flow, and puts stress on the adhesive bond. If temperature cycling is severe enough, then the bond between the lid and the substrate board can fail and/or the lid can delaminate from the thermal interface material resulting in degraded thermal performance of the system.
Significant warping due to thermal expansion against an edge constraint is a problem that does not present itself in the case of encapsulated microchip package configuration having heat spreader cap that lacks a lip that is bonded to the substrate. This is because in such encapsulated microchip package configurations the encapsulant layer between the between the cap and the substrate is so thick that it is relatively compliant in shear so that the substrate does not present an edge constraint to the thermal expansion of the cap.
Although changing the design to use a more compliant bond between the lip of a heat spreader lid and the substrate in non-encapsulated microchip packages might also serve to mitigate thermal stresses, doing so can bring other practical problems. Specifically, the use of soft adhesives in a precision manufacturing environment can unacceptably reduce control over the ultimate position of the components and/or the robustness of their attachment, and can cause unacceptable contamination, outgassing, etc. What is needed is a heat spreader lid having a lip that can be bonded to the substrate board by conventional methods that are used in typical non-encapsulated chip packages, yet that can also be thermally cycled with improved reliability.